A high-efficiency furnace typically employs several heat exchangers to warm an air stream passing through the furnace. The heat exchanger may include “clamshell” halves formed by stamping metal sheets, the halves being fastened together in a clamshell assembly to form a passageway for burning fuel and directing hot flue gas. The heat exchanger typically has a large temperature gradient when operating, placing the clamshell halves under significant stress.